cppgc: young-gen: Fix ubsan failures in minor gc tests

This fixes:
runtime error: upcast of misaligned address 0x000000000001
for type 'cppgc::internal::(anonymous namespace)::SimpleGCed<64>', which
  requires 8 byte alignment

Bug: chromium:1029379
Change-Id: Id03ce022e72fc07a18c171d4cf5e42f50cb684f1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3622918
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80332}

test/unittests/heap/cppgc/minor-gc-unittest.cc

@@ -346,7 +346,7 @@ TYPED_TEST(MinorGCTestForType, OmitGenerationalBarrierForSentinels) {
   {
     ExpectNoRememberedSlotsAdded _(*this);
     // Try issuing generational barrier for sentinel.
-    old->next = static_cast<Type*>(kSentinelPointer);
+    old->next = kSentinelPointer;
   }
 }